In most communication systems, increasing bandwidth translates into a greater transmission capacity. However, the physical medium or communication channel usually has some characteristics that vary in frequency, and only certain fragments or frequency bands can be used. For example, in an electrical grid, the power that can be injected starting from 30 MHz may vary in comparison to the power that can be injected at frequencies lower than 30 MHz. Also, an electrical grid has other characteristics that make an electrical grid less desirable as a transmission medium. For example, in an electrical grid, the noise floor decreases with the frequency, the attenuation increases with the frequency, and there is a lot of noise in the frequency-modulation (FM) radio band (from 88.5 to 108 MHz).
Various technologies related to multiband transmission exist within the state of the art. For example, patent WO2004/100392 shows a system with several overlapping sub-bands whose separation is accomplished digitally by means of a bank of digital filters. The process for utilizing a higher frequency band is known in the state of the art, and such process has a number of possible implementations, like those shown in U.S. Pat. No. 6,985,715 and US2002/0010870. These patents show how to go higher in frequency in order to use another frequency band. WO2007/039723 proposes several independent and different systems that work in different frequency bands having a different physical part (PHY) and medium access part (MAC) for each band. This has the drawback that the chains of transmission and reception must be duplicated at the digital level, a high cost of the analog separation filters and a larger guard band between the different systems. In US2008/006310, the concept of variable symbol time appears.